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Synthesis and optical properties of CdSe nanocrystals and CdSe/ZnS core/shell nanostructures in non-coordinating solvents

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Adv. Nat. Sci.: Nanosci. Nanotechnol. 1 (2010) 025004 (4pp)


Synthesis and optical properties of CdSe nanocrystals and CdSe/ZnS core/shell nanostructures in non-coordinating solvents Hong Quang Nguyen Department of Physics, Vinh University, 182 Le Duan Street, Vinh City, Vietnam E-mail: [email protected]

Received 27 February 2010 Accepted for publication 14 June 2010 Published 3 August 2010 Online at Abstract We have performed a conventional and non-coordinated-based method for synthesis of CdSe and CdSe/ZnS core/shell quantum dots in this work. This was the first time a CdSe/ZnS core/shell structure was successfully synthesized in a non-coordinating solvent without trioctylphosphine oxide (TOPO). The obtained CdSe nanocrystals were characterized by using UV-Vis absorption spectroscopy, photoluminescent (PL) spectroscopy and transmission electron microscopy (TEM), which confirmed that a series of CdSe particles with a diameter of 1.9–3.5 nm, corresponding to the first peak of absorption spectra in the 450–570 nm range, was successfully achieved. The CdSe/ZnS core/shell structures were then fabricated by coating the previously synthesized CdSe core with various ZnS layers. These CdSe/ZnS semiconductor quantum dots exhibited very high photoluminescence in comparison to that of the original CdSe cores. The narrow width of the CdSe/ZnS quantum dots indicated that the as-produced quantum dots have uniform size distribution, desirable dispersibility and excellent fluorescent properties. These are the requirements for several potential utilizations, such as cellular imaging, biomedical sensing, and solar cell and other photovoltaic applications. Keywords: CdSe/ZnS, core/shell, quantum dot, photoluminescence, synthesis Classification numbers: 4.00, 4.03, 5.04

exciting and challenging issues of CdSe QDs is how to make these nanocrystals in homodispersed size. Several methods of growing CdSe QDs have been reported [16–24]. The most common route for CdSe synthesis involves an organometallic precursor in a coordinating solvent [16, 25–28]. Typically, dimethyl cadmium is reacted with selenium in trioctylphosphine oxide (TOPO) at a relevant temperature, usually 300 ◦ C. Dimethyl cadmium is well known as extremely toxic, expensive, unstable, explosive and pyrophoric, making the system difficult to control or reproduce. Recently, Peng and Peng [29] have pioneered the kinetic synthesis of CdSe nanocrystals from CdO and elemental Se as an example of green chemistry, where relatively safe materials are used, although the hazards associated with the CdO and Se should not be overlooked. Based on this approach, Boatman et al [23] adopted an

1. Introduction Semiconductor nanocrystals, also known as quantum dots (QDs), have attracted considerable attention because of their unique size-dependent properties. Among them, CdSe nanocrystals are ideal building blocks for the formation of QD superlattices. Photoluminescent (PL) emission from colloidal CdSe QDs can be adjusted within the visible spectrum, from 475 to 670 nm. This versatility leads to numerous photonic applications, such as solar cells [1–3], optical fiber amplifiers [4], color displays using light-emitting-diode arrays [5–7], optical temperature probes [8, 9] and cellular imaging [10–15]. For these reasons, CdSe QDs have received considerable attention from the scientific community all over the world, especially developed countries. One of the most 2043-6254/10/025004+04$30.00


© 2010 Vietnam Academy of Science & Technology

Adv. Nat. Sci.: Nanosci. Nanotechnol. 1 (2010) 025004

H Q Nguyen

easier, safer and faster way of producing CdSe QDs. They used CdO as the Cd precursor and ODE, a non-coordinating solvent, to synthesize CdSe at 225 ◦ C. More recently, Park et al [30] reported a similar method for growing CdSe but at a higher temperature, 290 ◦ C. They stated that the nucleation and growth were very fast and completed within 100 s. Many others have also investigated the influence of the Cd/Se ratio [31] and growing temperature on the properties of the obtained CdSe QDs. One of the important characteristics of semiconductors concerns the influence of the surface on optical and electrical properties, and the need to embed semiconductor clusters in a passivating medium [32]. It is well known that the photo-luminescent (PL) emission intensity of CdSe QDs increases several times when the CdSe cores are capped inside a shell of high bandgap material like ZnS to form a CdSe/ZnS core-shell structure. Emission characteristics of CdSe/ZnS core-shell QDs are well known to depend strongly upon the size of the CdSe core. Changes in size and size distribution of the QDs can subsequently change the luminescence and optical properties of the as synthesized nanocrystals. Here we report on an approach for the synthesis of CdSe nanocrystals with CdO and ODE as Cd precursor and non-coordinating solvent, respectively. The influence of the CdSe growing time and the effect of a ZnS coating layer on the QD emission intensity are also discussed.

Figure 1. Absorbance spectra of CdSe QDs grown in various reaction times.

after centrifugation at 5000 rpm for 15 min. The solid precipitate was redissolved in about 10 ml hexane. For a ZnS overcoating on CdSe core nanocrystals, diethylzinc (ZnEt2) and hexamethyl-disilathiane ((TMS)2 S) were used as the Zn and S precursor, respectively. The amounts of Zn and S precursor needed to grow a ZnS layer of the desired thickness for each CdSe sample were determined based on the method proposed by Bawendi and co-workers [33]. Equivalent molar amounts of the Zn and S precursors (1 ml each) were dissolved in 2.6 ml of TOP inside an inert atmosphere glove-box, then loaded into a syringe for the shelling step. A portion of CdSe-ODE in hexane was transferred into a methanol–chloroform mixture (CH3 OH : CHCl3 , 1 : 1 in volume). The extraction of CdSe-ODE was heated to 150 ◦ C, and then a syringe containing 2 ml of a mixture of Zn and S precursors was slowly dropped into the CdSe solution for coating. Aliquots of CdSe/ZnS after 2 min, 5 min and 1 h of shelling were extracted from the flask to monitor the PL emission spectra and to compare with those of the CdSe core (annealing for solidification at 150 ◦ C, before shelling). After coating, the Cd/Se structures were kept at 150 ◦ C for the solidification of the shelling layer. Photoluminescence measurements were obtained using a fluorometer (Fluorolog-3 fluorometer, Horiba Jobin Yvon). Absorbance spectra of the samples were recorded using a spectrophotometer (Ocean Optic Spectrometer USB2000). Suspensions of the CdSe QDs in hexane were placed in a cuvette with polished sides. Transmission electron microscope (TEM) images were obtained with a Hitachi H-7000. High resolution images were taken using a Philips CM 300 FEM TEM. Samples for TEM were prepared by placing tiny drops of QD suspensions on a lacey carbon grid.

2. Experimental All the chemicals, including cadmium oxide (CdO), selenium (Se) powder (99.999 + %), trioctylphosphine (TOP, tech., 90%), oleic acid (OA, tech., 90%) and 1-octadecene (ODE, tech., 90%), were purchased from Aldrich. In the experiments, the TOP was used as the ligand of the Se precursor, and OA was chosen as the ligand of the Cd precursor in the ODE solvent. CdSe QDs with diameters ranging from 2.03 to 3.19 nm were grown using a wet chemical synthesis method as the modified synthesis [23]. First, 0.1 M solutions of the Cd and Se precursors were separately prepared. The stock solution of Se precursor was prepared by dissolving 150 mg Se powder in 20 ml TOP with a hot-stir plate for 2 h in a glove box with a nitrogen flux of 200 sccm. The Cd precursor was prepared by mixing 160 mg CdO, 3.5 ml oleic acid and 10 ml ODE at 200 ◦ C until the solution became colorless. The CdO–OA–ODE mixture was transferred to the hot flask and stirred continuously. The temperature was then increased to 280 ◦ C. When the solution became transparent, the temperature was decreased to 225 ◦ C. At this temperature, a syringe containing 5 ml TOP-Se solution was swiftly injected into the hot flask. We note that the temperature was easily achieved using a heating mantle. Small aliquots (typically about 1 ml each) were extracted from the mixture at different reaction times, from 10 s to about 20 min. These aliquots should be immediately cooled by being mixed with 1 ml toluene to monitor absorbance spectra. Next, 20 ml hexane (EMD, HPLC grade) was added to the remaining solution to interrupt the growth. After washing the remaining organics and solvent, acetone was added until the solution became opaque. The supernatant was discarded

3. Results and discussions The absorbance spectra as a function of reaction time after the injection of the Cd/Se precursor mixture are depicted in figure 1. Here, the absorption peaks appear on each curve, indicating that a series of CdSe QDs were formed 2

Adv. Nat. Sci.: Nanosci. Nanotechnol. 1 (2010) 025004

H Q Nguyen

Table 1. Average diameter of CdSe QDs determined from the obtained absorbance spectra. Absorbance peak (nm) Average diameter (nm)

463 2.05

472 2.11

480 2.17

487 2.23

Figure 2. Photoluminescence intensity of the CdSe crystals with a reaction time from 10 s to 20 min.

506 2.40

517 2.53

532 2.73

539 2.84

549 3.02

557 3.19

Figure 3. Photoluminescent intensity of CdSe/ZnS with various reaction times after annealing at 150 ◦ C.

in the solvent. The reaction time increases from 10 s to 20 min, and the absorption peak red-shifts from 463 up to 557 nm, sequentially. The absorption edge is relatively sharp, indicating a relatively narrow size distribution. From the peak positions, one can estimate the average size of CdSe QDs following the formula [34]:   D = 1.6122 × 10−9 λ4 − 2.6575 × 10−6 λ3  + 1.6242 × 10−3 λ2 − (0.4277) λ + 41.57, (1) where D is the average diameter of the CdSe QDs and λ is the wavelength of the absorption peak. From equation (1), the diameter of the obtained CdSe ranged from 2.03 to 3.19 nm, corresponding to the peak position of 463–557 nm. These estimated results are shown in table 1. The PL emission spectra of these CdSe are represented in figure 2, which shows a red-shift for each sample. From the spectra, it is clear that, if the time of reaction is short (< 180 s), there is a broad peak on the spectra, which might be attributed to the unstable state of the nucleation stage in this state. For longer shelling times, the PL intensity is reduced. It is suggested that a moderate growth time would lead to better results in the monodispersed size distribution. This result is consistent with previous work on CdSe QDs [35–37]. Figure 3 represents the PL spectra of the CdSe core and the CdSe/ZnS core/shell. It is clear that the PL emission of CdSe/ZnS improved significantly because of the passivation in the CdSe core by ZnS shelling layers. The PL intensity of the core/shell structure is about 10–15 times higher than that of the original CdSe core. The shelling time also affected the PL intensity. While a shelling for 5 min improved the PL intensity up to 15 times (CdSe/ZnS core-shell 5 min sample) compared to that of the CdSe core sample (the core-after 3 min), the CdSe/ZnS core/shell after 1 h had reduced its PL intensity. It is possible that coating for a long time resulted

Figure 4. Transmission electron micrograph of CdSe/ZnS crystals (annealed at 150 ◦ C).

in more layers of ZnS being formed. These layers reduced the CdSe/ZnS PL emission. From the experimental results, it is suggested that the ideal duration for shelling is about 5 min. Depending on the concentration of the CdSe QDs as well as the Cd/Se ratio, the shelling time may be altered accordingly. The transmission electron micrographs of CdSe before and after annealing at 150 ◦ C are shown in figure 4. The inset in figure 4 shows the HRTEM image of CdSe/ZnS. We can see a good agreement between the average size of CdSe determined from the absorption peak and the size shown on the TEM image. For smaller particles, the band gap enlargement was smaller than predicted by the model. Nonetheless, this correction provides a conventional tool for estimating the average particle diameter from the absorbance spectra without the need for measurement using TEM, at least for particle sizes from 2 to 4 nm, as in this case. 3

Adv. Nat. Sci.: Nanosci. Nanotechnol. 1 (2010) 025004

H Q Nguyen

4. Conclusion

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In this paper, we have reported on an approach for the synthesis CdSe QDs using CdO and DE as a Cd precursor and non-coordinating solvent, respectively. The synthesis was implemented at 225 ◦ C, a relatively low temperature in comparison with the previously reported methods. The estimation of the average diameter of CdSe based on the TEM image agreed very well with the calculation based on the absorbance spectra. Chemical passivation of CdSe by ZnS layers was essential to improve the PL emission of the nanostructures. This could increase the PL intensity up to 15 times compared to that of the original sample. In order to obtain the optimal PL intensity, the passivation process should not last too long. A solution procedure, relying on heterogeneous nucleation of the passivating layer on the surface of the nanocrystals, was developed for the passivation.

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